Installing on a ZyXel NSA325

I'm following the instructions for the device and seem to be falling at the first hurdle as I can not get able sensible output through the serial console. I've connected it as per the instructions but all I get is a stream of gibberish and only then if the GND isn't connected. If the GND is connected I get nothing.
( Using a Rasp PI with a FDTI 3v3 USB cable - console via screen at 115200 baud).

Any ideas or is one of the serial interfaces broken?

if not it'll have to be consigned to the recyler. I do hate throwing stuff away.

There is no NSA235, did you mean NSA325?
Did you connect VCC? If yes, REMOVE it. You only need to connect Tx, Rx and GND.

If that wasn't the problem, then try to swap Tx and Rx, they need to be crossed: FTDI Tx goes to NSA Rx and vice versa.

Yes NSA325. No VCC connected. swapped the RX and TX cables around several times. One way gives gibberish other way nothing but nothing works with the GND connected!
Retried it on a WIN11 box with PuTTY and it gives the same - GND, TX, RX nothing. TX (NSA)- RX( PC) loads of gibberish. I think it may be broken.

(typo in topic fixed)

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If there is gibberish, it's often either a bad (GND) connection or a speed mismatch. Or a fake (broken) TTL-USB converter. Or, of course, a broken board.

Did you try different baud rates and did you double-check all connections?

I think it's a broken board, dug out an other old TTL-USB converter (no longer supported in Windows) and used that with the PI and the result was was the same. I did try 9600, same result - as it doesn't seem to work with the GND plugged in I think the serial port is broken. The rest of it seems to work fine - pity there's no manufacture support for it anymore (or SMB2!).

Thanks for the suggestions I had tried them all but it always helps to have a someone else to help, in case you have missed something silly.